FIELD OF THE INVENTION

The present invention relates to a method for controlling yaw of a wind turbine with hinged wind turbine blades. More particularly, the method according to the invention allows a yaw moment and/or a yaw position and/or a yaw speed of the wind turbine to be adjusted in an easy and fast manner.

BACKGROUND OF THE INVENTION

Wind turbines are normally controlled in order to provide a desired power output and in order to control loads on the wind turbine. For horizontal axis wind turbines, i.e. wind turbines with a rotor which rotates about a substantially horizontal rotor axis, this may be obtained by controlling a pitch angle of the wind turbine blades. In this case the angle of attack of the wind turbine blades relative to the incoming wind is adjusted by rotating the wind turbine blades about a longitudinal axis.

As an alternative, wind turbines may be provided with wind turbine blades which are connected to a blade carrying structure via hinges, thereby allowing a pivot angle defined between the wind turbine blades and the blade carrying structure to be varied. In such wind turbines the diameter of the rotor of the wind turbine, and thereby the area swept by the rotor, is varied when the pivot angle is varied. An example of such a wind turbine is disclosed in US 4,632,637.

During operation of a wind turbine, the rotor of the wind turbine needs to be oriented correctly relative to the incoming wind, in order to avoid uneven loads on the wind turbine, in particular on the rotor and/or components connected thereto. More particularly, the rotor needs to be oriented directly towards the incoming wind, for upwind wind turbines, and directly opposition the incoming wind, for downwind wind turbines. Accordingly, when the wind direction changes, the orientation of the rotor needs to be adjusted accordingly. This is normally done by rotating a nacelle carrying the rotor, relative to a tower, by means of a yaw system. The yaw system may be an active yaw system in which the nacelle is actively rotated by means of yaw drives, or a passive yaw system in which the nacelle automatically rotates according to the wind direction. However, such traditional yaw systems may not react sufficiently fast to fast changes in the wind direction and/or to small fluctuations in the wind direction.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling yaw of a wind turbine, in which a yaw moment and/or a yaw position and/or a yaw speed of the wind turbine can be adjusted in a fast and easy manner.

According to a first aspect the invention provides a method for controlling yaw of a wind turbine, the wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and one or more wind turbine blades connected to the blade carrying structure via a hinge, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, the wind turbine further comprising an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade which biases the wind turbine blade towards a position defining minimum pivot angle or towards a position defining maximum pivot angle, the method comprising the steps of:

- detecting a yaw signal of the wind turbine,

- comparing the detected yaw signal to a reference yaw signal, and

- in the case that the difference between the detected yaw signal and the reference yaw signal exceeds a first predefined threshold value: monitoring azimuth position of each of the wind turbine blades, and - changing the biasing force applied to each wind turbine blade as a function of azimuth position for wind turbine blades in at least some azimuth positions at a first side of a vertical level intersecting the hub or for wind turbine blades in at least some azimuth positions at a second, opposite, side of the vertical level, thereby creating a yaw moment which causes the yaw signal of the wind turbine to approach the reference yaw signal.

Thus, the method according to the first aspect of the invention is a method for controlling yaw of a wind turbine. In the present context the term 'wind turbine' should be interpreted to mean a construction which is capable of extracting energy from the wind and transforming it into electrical energy. In the present context the term 'yaw' should be interpreted to mean the orientation of the nacelle, and thereby the rotor, of the wind turbine, relative to the wind direction.

The wind turbine comprises a tower, a nacelle, a hub and one or more wind turbine blades. The nacelle is mounted on the tower via a yaw system, thereby allowing the nacelle to be rotated relative to the tower in order to direct the wind turbine blades in accordance with the direction of the wind. The yaw system may be an active yaw system in which the nacelle is rotated actively by means of a yaw drive mechanism, e.g. on the basis of measurements of the wind direction. As an alternative, the yaw system may be a passive yaw system in which the nacelle automatically rotates according to the wind direction without the use of a yaw drive mechanism. As another alternative, the yaw system may be a combination of an active yaw system and a passive yaw system, in the sense that it may operate actively under some circumstances and passively under other circumstances.

The wind turbine is preferably a downwind wind turbine, i.e. a wind turbine in which the incoming wind passes the nacelle and the tower before reaching the rotor.

The hub comprises a blade carrying structure, and the wind turbine blades are connected to the blade carrying structure via a hinge. Thereby each of the wind turbine blades is arranged to perform pivot movements relative to the blade carrying structure, via the hinge. A pivot angle is thereby defined between each wind turbine blade and the blade carrying structure, depending on the position of the hinge and thereby of the wind turbine blade relative to the blade carrying structure. Accordingly, the pivot angle defines the direction along which a given wind turbine blade extends relative to the blade carrying structure, and thereby relative to the hub. This, in turn, determines the diameter of the rotor, and thereby the ability of the wind turbine to extract energy from the wind.

The pivot angle can vary between a minimum pivot angle, defining a maximum or near maximum rotor diameter, and a maximum pivot angle, defining a minimum or near minimum rotor diameter. Positioning the wind turbine blades at maximum pivot angle is sometimes referred to as 'barrel mode'.

Thus, the wind turbine is of a kind which comprises hinged wind turbine blades. The hinge may be or comprise a bearing, e.g. in the form of a journal bearing, a roller bearing, or any other suitable kind of bearing.

The hub is mounted rotatably on the nacelle. Since the wind turbine blades are mounted on the hub, they rotate along with the hub, relative to the nacelle.

The wind turbine further comprises an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade. The biasing force biases the wind turbine blades towards a position defining minimum pivot angle or towards a position defining maximum pivot angle.

In the case that the biasing mechanism biases the wind turbine blades towards a position defining minimum pivot angle, pivot movements of the wind turbine blades towards larger pivot angles are performed against the biasing force. Furthermore, if no other forces act on the wind turbine blades, the biasing force will cause the wind turbine blades to be positioned at the minimum pivot angle.

Similarly, in the case that the biasing mechanism biases the wind turbine blades towards a position defining maximum pivot angle, pivot movements of the wind turbine blades towards smaller pivot angles are performed against the biasing force. Furthermore, if no other forces act on the wind turbine blades, the biasing force will cause the wind turbine blades to be positioned at the maximum pivot angle, i.e. in barrel mode.

In any event, the pivot angle of the wind turbine blades is defined by the biasing force applied thereto in combination with any other forces, such as aerodynamic forces, centrifugal forces, thrust forces originating from wind pressure, etc.

Since the biasing force is adjustable, it is possible to adjust how large an oppositely directed force is required in order to move the wind turbine blades away from the minimum pivot angle or the maximum pivot angle.

The biasing force could, e.g., be applied by means of wires attached to the inner blade parts or the outer blade parts of the wind turbine blades, which pull the wind turbine blades outwards, i.e. towards the minimum pivot angle, or inwards, i.e. towards the maximum pivot angle. In this case the biasing force can be adjusted by adjusting the pulling force applied by the wires.

As an alternative, the biasing force could be applied by means of one or more springs acting on the wind turbine blades, e.g. compressible springs arranged for pulling or pushing the wind turbine blades towards the minimum pivot angle or towards the maximum pivot angle. In this case the biasing force can, e.g., be adjusted by means of pulleys or hydraulic actuators mounted in the hub, in the blade carrying structure, in the wind turbine blade itself, in the nacelle or in the tower.

As another alternative, the biasing force could be in the form of a moment. In this case the biasing force could be applied by means of a torsional spring arranged in the hinge which pulls or pushes the wind turbine blades towards the minimum pivot angle or towards the maximum pivot angle. In this case the biasing force may also be adjusted by varying the torsional moment, e.g. by means of pulleys or hydraulic actuators mounted in the hub, in the blade carrying structure, in the wind turbine blade itself, in the nacelle or in the tower.

As another alternative, the biasing force could be applied by means of hydraulic mechanisms connected to the wind turbine blades and being arranged for pulling or pushing the wind turbine blades towards the minimum pivot angle or towards the maximum pivot angle. In this case the biasing force can be adjusted by adjusting the pressure in the hydraulic mechanisms.

In the method according to the invention a yaw signal of the wind turbine is initially detected. The detected yaw signal could, e.g., be or represent a yaw moment which is actually acting on the wind turbine, and may thereby be an indication of whether or not and to which extent forces are acting in the wind turbine which tends to urge the wind turbine to perform yawing movements in a given direction. The detected yaw signal may represent a zero yaw moment, in which case no forces tend to urge the wind turbine to perform yawing movement. Alternatively, the detected yaw signal may represent a non-zero yaw moment, in which case the wind turbine is in fact subjected to such forces. A yaw moment may, e.g., be detected by direct measurement, e.g. using strain measurements in rotor parts or nacelle parts. Alternatively, a yaw moment may be detected in an indirect manner, such as by means of the biasing mechanism, e.g. measuring a tension in the biasing mechanism, or by means of an active yaw system.

As an alternative to detecting a yaw moment, the detected yaw signal could be a signal related to a yaw error, a yaw speed, a yaw position and/or any other suitable parameter.

Next, the detected yaw signal is compared to a reference yaw signal.

Accordingly, it is investigated whether or not the detected yaw signal is as expected or desired, i.e. whether or not it is equal to the reference yaw signal. If the detected yaw signal is equal to, or almost equal to, the reference yaw signal, it can be assumed that the nacelle is in the correct yaw position, or that the nacelle is performing an intended yaw movement, and therefore nothing further is done.

On the other hand, in the case that the difference between the detected yaw signal and the reference yaw signal exceeds a predefined threshold value, this is an indication that the detected yaw signal differs significantly from the reference yaw signal, and thereby from an expected and desired yaw signal. This may, e.g., have the consequence that a yaw moment acting on the wind turbine causes undesired loads on the wind turbine, in particular uneven loads, e.g. on the rotor. Therefore, in this case a process for controlling the yaw of the wind turbine is initiated.

To this end an azimuth position of each of the wind turbine blades is monitored. In the present context the term 'azimuth position' should be interpreted to mean the position of a wind turbine blade along the rotating direction of the hub. Thus, the azimuth position of a wind turbine blade defines an angular direction of the wind turbine blade projected into a plane perpendicular to the direction of the main shaft.

Furthermore, the biasing force applied to each wind turbine blade is changed as a function of azimuth position, i.e. the change or adjustment of the biasing force applied to a given wind turbine blade is dependent on the momentary azimuth position of that wind turbine blade. Furthermore, the change of the biasing force applied to a given wind turbine blade varies as that wind turbine blade rotates along with the hub, and the azimuth position of that wind turbine blade thereby changes. Finally, the change of the biasing force applied to the wind turbine blades at a given point in time will vary from one wind turbine blade to another, because the wind turbine blades will, at any time, be positioned at different azimuth positions.

The change of the biasing force applied to each wind turbine blade is performed in such a manner that either the biasing force applied to wind turbine blades in at least some azimuth positions at a first side of a vertical level intersecting the hub is changed, or the biasing force applied to wind turbine blades in at least some azimuth positions at a second, opposite, side of the vertical level is changed.

Thereby a difference in applied biasing force is introduced between the part of the rotor plane arranged at the first side of the vertical level and the part of the rotor plane arranged at the second side of the vertical level. This may result in a difference in pivot angles at the first and second sides of the vertical level, respectively, and/or a difference in forces acting between the wind turbine blades, the blade carrying structure and the biasing mechanism at the first and second sides of the vertical level, respectively. This difference creates a yaw moment which causes the yaw signal of the wind turbine, e.g. a detected yaw moment, to approach the reference yaw signal.

Thus, the operation of the biasing mechanism in order to change the biasing force applied to the wind turbine blades as a function of azimuth position is used for obtaining a desired yaw moment, a desired yaw speed and/or a desired yaw position. This is an easy manner of ensuring that an appropriate yaw position is reached in an appropriate manner, e.g. fast or with a suitable yaw speed, and this approach is able to react fast to changes or fluctuations in wind direction.

For instance, the change of the biasing force applied to each of the wind turbine blades may be performed in such a manner that the pivot angle for wind turbine blades in at least some azimuth positions at the first side of the vertical level is increased or in such a manner that the pivot angle for wind turbine blades in at least some azimuth positions at the second side of the vertical level is decreased.

In this case, the biasing force applied to the wind turbine blades is such that, if no other forces were applied to the wind turbine blades, the pivot angle of wind turbine blades extending at the first side of the vertical level is larger than the pivot angle of wind turbine blades extending at the second side of the vertical level.

For instance, if the biasing force applied to the wind turbine blades biases the wind turbine blades towards a position defining minimum pivot angle, then the biasing force applied to the wind turbine blades extending at the first side of the vertical level may be decreased or the biasing force applied to the wind turbine blades extending at the second side of the vertical level may be increased.

Similarly, if the biasing force applied to the wind turbine blades biases the wind turbine blades towards a position defining maximum pivot angle, then the biasing force applied to the wind turbine blades extending at the first side of the vertical level may be increased or the biasing force applied to the wind turbine blades extending at the second side of the vertical level may be decreased. When the biasing force applied to the wind turbine blades is changed as described above, the rotor area defined by the wind turbine blades which extend at the first side of the vertical level is smaller than the rotor area defined by the wind turbine blades extending at the second side of the vertical level. Therefore, the thrust on the wind turbine blades originating from the incoming wind is higher for the wind turbine blades which extend at the second side of the vertical level than for wind turbine blades which extend at the first side of the vertical level. This difference in thrust creates the yaw moment. A yaw moment in an opposite direction can be created in a similar manner by changing the biasing force applied to each of the wind turbine blades in such a manner that the pivot angle for wind turbine blades in at least some azimuth positions at the first side of the vertical level is decreased or in such a manner that the pivot angle for wind turbine blades in at least some azimuth positions at the second side of the vertical level is increased.

Furthermore, in the case that the biasing mechanism is of a kind which pulls the wind turbine blades towards the blade carrying structure, the force applied to the blade carrying structure by the biasing mechanism may further contribute to creating the yaw moment.

The step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force for wind turbine blades in at least some azimuth positions at the first side of the vertical level in a first direction and changing the biasing force for wind turbine blades in at least some azimuth positions at the second side of the vertical level in a second, opposite direction.

According to this embodiment, the biasing force applied to each wind turbine blade is changed at the first side of the vertical level as well as at the second side of the vertical level. However, the biasing force is changed in opposite directions at the first and second sides of the vertical level, respectively. Thereby the difference in biasing force applied to the wind turbine blades at the first side of the vertical level and the second side of the vertical level, respectively, is still introduced. Furthermore, a larger difference, and thereby a larger created yaw moment, can be obtained without having to introduce large changes in applied biasing force in one of the half planes of the rotor. For instance, the biasing force applied to wind turbine blades at azimuth positions at the first side of the vertical level may be decreased, while the biasing force applied to wind turbine blades at azimuth positions at the second side of the vertical level may be increased. Alternatively, the biasing force applied to wind turbine blades at azimuth positions at the first side of the vertical level may be increased, while the biasing force applied to wind turbine blades at the second side of the vertical level may be decreased.

The detected yaw signal may be a detected yaw moment and the reference yaw signal may be a zero yaw moment, and the created yaw moment may counteract the detected yaw moment.

According to this embodiment, a yaw moment is detected and compared to a reference yaw signal in the form of a zero yaw moment. Thus, the wind turbine is not supposed to actively perform yawing movements, but is rather supposed to remain in a given yaw position and/or to passively align with the wind direction. However, something is acting on the wind turbine and introducing a yaw moment which is not supposed to be there. This may cause undesired uneven loads on the wind turbine. Therefore, the created yaw moment caused by changing the biasing force applied to the wind turbine blades as a function of azimuth position is created in such a manner that the detected yaw moment is counteracted, thereby reducing the risk of uneven loads on the wind turbine.

This is particularly relevant for wind turbines with passive yaw systems.

As an alternative, the detected yaw signal may be a detected yaw moment and the reference yaw signal may be a non-zero reference yaw moment, and the created yaw moment may increase the detected yaw moment in the case that the detected yaw moment is lower than the reference yaw moment, and the additional yaw moment may decrease the detected yaw moment in the case that the detected yaw moment is higher than the reference yaw moment.

According to this embodiment, a yaw moment is detected and compare to a reference yaw signal in the form of a non-zero reference yaw moment. Thus, in this case the wind turbine is supposed to be performing yawing movements. However, if it is established that the detected yaw moment differs from the reference yaw moment, then the yawing movements are either performed too slowly or too fast.

Thus, in the case that the detected yaw moment is lower than the reference yaw moment, this is an indication that the yawing movements are performed too slowly, and therefore the created yaw moment is, in this case, created in such a manner that the detected yaw moment increases, i.e. approaches the reference yaw moment, in order to increase the yawing speed.

Similarly, in the case that the detected yaw moment is higher than the reference yaw moment, this is an indication that the yawing movements are performed too fast, and therefore the created yaw moment is, in this case, created in such a manner that the detected yaw moment decreases, i.e. approaches the reference yaw moment, in order to reduce the yawing speed.

The step of detecting a yaw signal may comprise detecting a yawing speed of the wind turbine, and, in the case that the detected yawing speed exceeds a predefined speed threshold, the step of changing the biasing force applied to each wind turbine blade may be performed in order to reduce the yawing speed of the wind turbine.

According to this embodiment, apart from or in addition to monitoring a yaw moment, the yawing speed of the wind turbine is monitored. If the wind turbine is yawed at a yawing speed which is too high, there is a risk of high loads on the wind turbine. Therefore, if the yawing speed exceeds a predefined speed threshold, the changing of the biasing force applied to the wind turbine as a function of azimuth position described above is initiated. Thereby a yaw moment is created, which counteracts the yawing movement, and thereby reduces the yawing speed.

A high yawing speed may, e.g., occur when a large change in wind direction occurs. Then a passive yaw system may attempt to yaw the wind turbine to be aligned with the new wind direction fast, resulting in a high yawing speed. Furthermore, it may be desirable to reduce the yawing speed in the case of high wind speeds. For instance, at high wind speeds the yawing speed may be limited to a certain maximum yawing speed, e.g. 0.5°/s.

The method may further comprise the step of, in the case that the difference between the detected yaw signal and the reference yaw signal exceeds a second threshold value, where the second threshold value is higher than the first threshold value, moving all of the wind turbine blades towards maximum pivot angle by changing the biasing force applied to each wind turbine blade.

According to this embodiment, in the case that the difference between the detected yaw signal and the reference yaw signal increases even further, i.e. to a level above the second threshold value, there may be a risk of damage to the wind turbine due to the loads introduced by the high deviation of the yaw signals. Therefore, in order to protect the wind turbine, all of the wind turbine blades are moved towards maximum pivot angle, i.e. towards barrel mode, where the impact on the wind turbine blades from the incoming wind is minimal, due to the small rotor area.

Furthermore, the step of changing the biasing force applied to each wind turbine blade as a function of azimuth position, thereby creating a yaw moment which causes the yaw signal of the wind turbine to approach the reference yaw signal, may be performed after the wind turbine blades have been moved towards maximum pivot angle.

Thus, according to this embodiment, when the detected yaw signal is far off with respect to the reference yaw signal, the rotor area is first reduced significantly in order to minimise the impact from the wind as described above. While the wind turbine blades are in this position, the yaw adjustment by changing the biasing force applied to the wind turbine blades is performed. Thereby the detected yaw signal can be adjusted to a value which is significantly closer to the reference yaw signal without risking excessive loads on the wind turbine. Once the difference between the detected yaw signal and the reference yaw signal has been reduced to an acceptable level, the pivot angle of the wind turbine blades may once again be decreased, thereby increasing the rotor area to a normal size.

The method may further comprise the step of detecting a wind direction, and the step of detecting a yaw signal of the wind turbine may be performed in the case that the detected wind direction is within a predefined wind sector.

For instance, it may be known that if the wind direction is within one or more specified wind sectors, there is an increased risk of the wind turbine experiencing large yaw moments. For instance, the wake conditions at the position of the wind turbine may vary significantly depending on the wind direction. For instance, in some wind sectors, it may be known that the wind turbine is subjected to half wake, i.e. a part of the rotor is in wake and another part is not. This introduces a yaw moment in the wind turbine. Therefore, when the wind direction is within such a wind sector, the step of detecting a yaw signal of the wind turbine is initiated, e.g. in order to monitor whether or not the yaw moment caused by the half wake increases above an undesired level.

Thus, according to this embodiment, the yaw signal is only monitored when the wind direction is within a wind sector in which large yaw moments, etc., are expected.

As an alternative to detecting the wind direction, a yaw position of the wind turbine may be detected in order to determine whether or not the wind direction is within a specified wind sector. This may, e.g., be relevant for wind turbines with passive yaw systems.

The step of detecting a yaw signal may comprise detecting a yaw error of the wind turbine, and the steps of monitoring azimuth position of each of the wind turbine blades and changing the biasing force applied to each wind turbine blade may be performed in the case that the detected yaw error exceeds a predefined yaw error threshold.

According to this embodiment, the steps of monitoring azimuth position and changing the biasing force are initiated based on a detected yaw error rather than based on a detected yaw moment. A large yaw error may be expected to result in a large yaw moment, because this introduces a difference in the thrust originating from the wind on the two half planes of the rotor.

The step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force in order to obtain a local maximum in created yaw moment at an azimuth position in which the wind turbine blade points directly in a horizontal direction.

According to this embodiment, the obtained created yaw moment varies as a function of azimuth position. Furthermore, when the wind turbine blades point directly in a horizontal direction, the created yaw moment is at a maximum level. Thereby the maximum created yaw moment is provided directly in a yawing direction, and the reference yaw signal, e.g. in the form of a reference yaw moment, can thereby efficiently be reached.

For instance, the step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force in order to obtain a local maximum in pivot angle and/or a local minimum in biasing force at an azimuth position in which the wind turbine blade points directly in a horizontal direction at the first side of the vertical level.

According to this embodiment, the rotor area defined by the wind turbine blades and/or the impact on the wind turbine blades and/or the blade carrying structure from the biasing mechanism is/are minimal when the wind turbine blades point directly in a horizontal direction at the first side of the vertical level. Thereby the created yaw moment is created in such manner that it attempts to yaw the nacelle towards the second side of the vertical level, in the case that the wind turbine is an upwind wind turbine, and towards the first side of the vertical level, in the case that the wind turbine is a downwind wind turbine.

For instance, the biasing force may be changed in such a manner that as soon as a wind turbine blade reaches an azimuth position at the vertical level, thereby entering the region at the first side of the vertical level, a change is initiated which causes a gradual increase in the pivot angle and/or a gradual decrease in the biasing force. This continues until the wind turbine blade reaches the azimuth position in which the wind turbine blade points directly in the horizontal direction. Then the biasing force is changed in order to cause a gradual decrease in the pivot angle and/or a gradual increase in biasing force, at least until the wind turbine blade reaches an azimuth position at the vertical level. The change in the biasing force may, e.g., follow a sinus curve, or another cyclic pattern, as a function of azimuth position.

As an alternative, the change of the biasing force may only be performed while the wind turbine blade is within a narrow range of azimuth positions around the azimuth position in which the wind turbine blade points directly in the horizontal direction at the first side of the vertical level.

Alternatively, the step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force in order to obtain a local maximum in pivot angle and/or a local minimum in biasing force immediately before the wind turbine blade reaches an azimuth position in which the wind turbine blade points directly in a horizontal direction at the first side of the vertical level.

According to this embodiment, the change of the biasing force in order to obtain minimal rotor area defined by the wind turbine blades and/or minimal impact from the biasing mechanism is performed immediately before the wind turbine blades reach the azimuth angle in which the wind turbine blades point directly in the horizontal direction at the first side of the vertical level.

When changing the biasing force applied to the wind turbine blades, there may be a phase delay before maximum created yaw moment is obtained, e.g. due to gyroscopic effects and/or flexibility of the wind turbine blades and the hinges. Therefore, by performing the change as described above, this delay is taken into account, and it is ensured that the minimal rotor area and/or minimal impact from the biasing mechanism is in fact obtained when the wind turbine blade reaches the azimuth position in which the wind turbine blade points directly in the horizontal direction at the first side of the vertical level. Alternatively or additionally, the step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force in order to obtain a local minimum in pivot angle and/or a local maximum in biasing force at an azimuth position in which the wind turbine blade points directly in a horizontal direction at the second side of the vertical level.

According to this embodiment, the rotor area defined by the wind turbine blades and/or the impact on the wind turbine blades and/or the blade carrying structure from the biasing mechanism is maximal when the wind turbine blades point directly in a horizontal direction at the second side of the vertical level. This will also result in a created yaw moment which attempts to yaw nacelle towards the second side of the vertical level for upwind wind turbines and towards the first side of the vertical level for downwind wind turbines, similarly to the embodiment described above.

Similarly to what is described above, the change of the biasing force may be initiated as soon as a wind turbine blade reaches an azimuth position at the vertical level, thereby entering the region at the second side of the vertical level, or only while the wind turbine blade is within a narrow range of azimuth positions around the azimuth position in which the wind turbine blade points directly in the horizontal direction at the second side of the wind turbine.

Alternatively, the step of changing the biasing force applied to each wind turbine blade may comprise changing the biasing force in order to obtain a local minimum in pivot angle and/or a local maximum in biasing force immediately before the wind turbine blade reaches an azimuth position in which the wind turbine blade points directly in a horizontal direction at the second side of the vertical level.

According to this embodiment, the change of the biasing force in order to obtain maximum rotor area defined by the wind turbine blades and/or maximum impact from the biasing mechanism is/are performed immediately before the wind turbine blades reach the azimuth angle in which the wind turbine blades point directly in a horizontal direction at the second side of the vertical level. Similarly to what is described above, a possible delay in obtaining maximum created yaw moment is thereby taken into account, thereby ensuring the maximum rotor area and/or maximum impact from the biasing mechanism is in fact obtained when the wind turbine blade reaches the azimuth position in which the wind turbine blade points directly in a horizontal direction at the second side of the vertical level.

It should be noted, that in order to obtain a created yaw moment which attempts to yaw the nacelle of an upwind wind turbine towards the first side of the vertical level, or a downwind wind turbine towards the second side of the vertical level, the embodiments described above may be reversed, in the sense that the biasing force may be changed in order to obtain a local maximum in pivot angle and/or a local minimum in biasing force at an azimuth position in which the wind turbine blade points directly in a horizontal direction at the second side of the vertical level, and/or the biasing force may be changed in order to obtain a local minimum in pivot angle and/or a local maximum in biasing force at an azimuth position in which the wind turbine blade points directly in a horizontal direction at the first side of the vertical level.

According to a second aspect the invention provides a wind turbine, the wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and one or more wind turbine blades connected to the blade carrying structure via a hinge, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, the wind turbine further comprising an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade which biases the wind turbine blade towards a position defining minimum pivot angle or towards a position defining maximum pivot angle, wherein the wind turbine is configured to control yaw in accordance with a method according to the first aspect of the invention.

Thus, the wind turbine according to the second aspect of the invention is a wind turbine with hinged blades, as described in detail above with reference to the first aspect of the invention. Furthermore, the wind turbine is configured to control yaw of the wind turbine in the manner described above with reference to the first aspect of the invention. Accordingly, the remarks set forth above with reference to the first aspect of the invention are equally applicable here.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Figs. 1-3 illustrate a wind turbine being controlled in accordance with a method according to a first embodiment of the invention,

Figs. 4-6 illustrate a wind turbine being controlled in accordance with a method according to a second embodiment of the invention,

Figs. 7 and 8 show details of a biasing mechanism for use in a method according to an embodiment of the invention, and

Fig. 9 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figs. 1-3 illustrate a wind turbine 1 being controlled in accordance with a method according to a first embodiment of the invention. Fig. 1 is a front view of the wind turbine 1, and Figs. 2 and 3 are side views of the wind turbine 1.

The wind turbine 1 of Figs. 1-3 comprises a tower 2 and a nacelle 7 mounted on the tower 2. A hub 3 is mounted rotatably on the nacelle 7, the hub 3 comprising a blade carrying structure 4 with three arms. A wind turbine blade 5 is connected to each of the arms of the blade carrying structure 4 via a hinge 6. Thus, the wind turbine blades 5 rotate along with the hub 3, relative to the nacelle 7, and the wind turbine blades 5 can perform pivoting movements relative to the blade carrying structure 4, via the hinges 6. Each wind turbine blade 5 defines an aerodynamic profile extending along the length of the wind turbine blade 5 between an inner tip end 5a and an outer tip end 5b. The hinge 6 is arranged at a hinge position of the wind turbine blade 5, the hinge position being at a distance from the inner tip end 5a as well as at a distance from the outer tip end 5b. The wind turbine blades 5 of the wind turbine 1 of Figs. 1-3 are straight in the sense that an inner portion of the wind turbine blade 5, between the hinge 6 and the inner tip end 5a, and an outer portion of the wind turbine blade 5, between the hinge 6 and the outer tip end 5b, extend along the same direction, i.e. an angle is not formed between the inner and outer portions of the wind turbine blade 5.

A biasing mechanism comprising wires 8 attached to the wind turbine blades 5 at a position near the inner tip end 5a applies a biasing force to the wind turbine blades 5 which pulls the wind turbine blades 5 towards a position defining minimum pivot angle, and thereby maximum rotor diameter. This will be described in further detail below with reference to Figs. 7 and 8.

In Fig. 2 the wind turbine blades 5 are positioned at the minimum pivot angle, i.e. at a pivot angle which results in a maximum rotor diameter of the wind turbine 1.

In Fig. 3 the wind turbine blades 5 are positioned at a larger pivot angle P than the minimum pivot angle illustrated in Fig. 2. Thereby the rotor diameter of the wind turbine 1 is smaller in the situation illustrated in Fig. 3 than in the situation illustrated in Fig. 2.

The wind turbine 1 of Figs. 1-3 is a downwind wind turbine, i.e. the wind direction relative to the wind turbine 1 is illustrated by arrow 9 shown in Figs. 2 and 3.

The wind turbine 1 of Figs. 1-3 may be operated in the following manner. Initially, a yaw signal, in the form of a yaw moment 10 of the wind turbine 1, is detected, and the detected yaw moment 10 is compared to a reference yaw signal, in the form of a reference yaw moment. In the case that the wind turbine 1 is not supposed to perform yawing movements, the reference yaw moment is zero. In the case that the wind turbine 1 is supposed to perform yawing movement, the reference yaw moment is a non-zero yaw moment which results in a desired yawing movement.

The detected yaw moment may originate from various sources, such as a changing wind direction, wake conditions, e.g. half wake, gusty or turbulent wind conditions, aerodynamic effects, gyroscopic effects, forces applied by the wires 8 to the blade carrying structure 4, etc.

A yaw moment 10 which differs from a desired or reference yaw moment by a certain amount may have a detrimental impact on some of the components of the wind turbine 1, notably the main shaft and the main bearing. Therefore, in some cases it may be necessary to reduce the difference between a detected yaw moment 10 and a reference yaw moment.

Therefore, in the case that the difference between the detected yaw moment 10 and the reference yaw moment exceeds a first predefined threshold value, a procedure is initiated in order to cause the detected yaw moment 10 to approach the reference yaw moment, and thereby decrease this difference, at least to a level below the first predefined threshold value.

To this end, the azimuth position of each of the wind turbine blades 5 is monitored. In Fig. 1 it can be seen that one of the wind turbine blades 5 is in an azimuth position in which the wind turbine blade 5 points directly upwards. The other two wind turbine blades 5 are in azimuth positions in which they extend in a downwards direction, but not directly downwards, and they extend on opposite sides of a vertical level intersecting the hub 3. Thus, one of the wind turbine blades 5 is in an azimuth position at a first side of a vertical level intersecting the hub, one of the wind turbine blades 5 is in an azimuth position at a second side of the vertical level, and one of the wind turbine blades is in an azimuth position which coincides with the vertical level. It should, however, be noted that since the hub 3 rotates, the azimuth position of each of the wind turbine blades 5 changes as a function of time. The biasing force applied to each of the wind turbine blades 5 by the wires 8 of the biasing mechanism is then changed as a function of azimuth position of the wind turbine blades 5. This is done in such a manner that the biasing force applied to wind turbine blades 5 in azimuth positions at the first side of the vertical level differs from the biasing force applied to the wind turbine blades 5 at the second side of the vertical level.

In the wind turbine 1 of Figs. 1-3, this is done by decreasing the pulling force applied by the wires 8 to wind turbine blades 5 at the first/second side of the vertical level and/or increasing the pulling force applied by the wires 8 to the wind turbine blades 5 at the second/first side of the vertical level. Accordingly, the pulling force applied to a given wind turbine blade 5 will vary as the wind turbine blade 5 rotates along with the hub 3, and its azimuth position therefore changes.

Thereby the rotor area defined by the wind turbine blades 5 at the first/second side of the vertical level becomes smaller than the rotor area defined by the wind turbine blades at the second/first side of the vertical level. The thrust applied to the wind turbine blades 5 by the wind 9 is therefore larger at the second/first side of the vertical level than at the first/second side of the vertical level. This creates an additional yaw moment which causes the total yaw moment 10 of the wind turbine 1 to approach the reference yaw moment.

Figs. 4-6 illustrate a wind turbine 1 being controlled in accordance with a method according to a second embodiment of the invention. The wind turbine 1 of Figs. 4-6 is very similar to the wind turbine 1 of Figs. 1-3, and will therefore not be described in detail here. Furthermore, the wind turbine 1 of Figs. 4-6 is controlled essentially as the wind turbine 1 of Figs. 1-3. Figs. 4-6 are all side views of the wind turbine 1 with the wind turbine blades 5 arranged at three different pivot angles. Fig. 4 shows the wind turbine blades 5 at minimum pivot angle, Fig. 6 shows the wind turbine blades 5 at maximum pivot angle, or barrel mode, and Fig. 5 shows the wind turbine blades 5 at an intermediate pivot angle. The wind turbine blades 5 of the wind turbine 1 of Figs. 4-6 are angled in the sense that the inner portion and the outer portion of the wind turbine blade 5 extend from the hinge 6 along different directions, forming an angle there between.

Figs. 7 and 8 show details of a biasing mechanism for applying a biasing force to wind turbine blades 5 of a wind turbine, e.g. the wind turbine 1 of Figs. 1-3 or the wind turbine 1 of Figs. 4-6.

Fig. 7 shows a portion of a blade carrying structure 4 and a portion of a wind turbine blade 5. The wind turbine blade 5 is pivotally mounted on the blade carrying structure 4 via a hinge (not shown). A wire 8 is connected to the wind turbine blade 5 at a position between an inner tip end 5a of the wind turbine blade 5 and the position of the hinge. The wire 8 extends from the connecting position at the wind turbine blade 5, via a pulley 11 and along the blade carrying structure 4 towards a hub (not shown).

A biasing force applied by means of the wire 8 pulls the wind turbine blade 5 towards a position defining a minimum pivot angle. In Fig. 7 the wind turbine blade 5 is arranged at the minimum pivot angle. Reducing the biasing force applied by means of the wire 8 will allow the wind turbine blade 5 to more easily pivot towards larger pivot angles.

Fig. 8 is a cross sectional view of part of a hub 3 and part of a nacelle 7. Arms of a blade carrying structure 4 are mounted on the hub 3. The wires 8 which are also illustrated in Fig. 7 are connected to a winch mechanism 12 arranged in the hub 3. Thereby the biasing force applied by means of the wires 8 can be adjusted by rotating the winch mechanism 12, thereby adjusting the length of the wires 8.

Fig. 9 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 13. At step 14 a yaw moment of the wind turbine is detected and compared to a reference yaw moment. At step 15 it is investigated whether or not the difference between the detected yaw moment and the reference yaw moment exceeds a predefined threshold value. If this is not the case, the process is returned to step 14 for continued monitoring of the yaw moment.

In the case that step 15 reveals that the difference between the detected yaw moment and the reference yaw moment exceeds the predefined threshold value, the process is forwarded to step 16, where yaw moment adjustment is initiated, and then to step 17, where azimuth dependent adjustment of the biasing force applied to the wind turbine blades is performed, in the manner described above. Thereby an additional yaw moment is created, which causes the total yaw moment of the wind turbine to approach the reference yaw moment. At step 18 it is investigated whether or not the total yaw moment of the wind turbine has approached the reference yaw moment sufficiently to cause the difference between the detected yaw moment and the reference yaw moment to decrease below the predefined threshold value. If this is not the case, the process is returned to step 17 for continued azimuth dependent adjustment of the biasing force applied to the wind turbine blades.

In the case that step 18 reveals that the difference between the detected yaw moment and the reference yaw moment is now below the predefined threshold value, the process is forwarded to step 19, where yaw moment adjustment is discontinued. Finally, the process is returned to step 14 in order to monitor the yaw moment.

It should be noted that a similar process to the on described above with reference to Fig. 9 could be performed based on a measured yaw error or a measured yawing speed.